For many people, coffee is the first thing that happens in the morning, often before any food. Online advice about this habit ranges from harmless to alarming: coffee on an empty stomach is said to cause ulcers, damage the stomach lining, spike stress hormones, and wreck blood sugar. Very little of that advice cites anything. This article goes through the claims one at a time and shows which have actual studies behind them, and what those studies found. The short version is that a few effects are real and measurable, but the most frightening claims are not supported.
Claim one: coffee raises stomach acid
This part is true. The classic experiment was published in the New England Journal of Medicine by Cohen and Booth[1], who compared the stomach acid response to caffeine, regular coffee and decaffeinated coffee in healthy volunteers. Regular coffee produced a maximum acid output of 20.9 mEq per hour, and decaf produced 16.5 mEq per hour. Pure caffeine, compared on a cup-equivalent basis, produced only 8.4.
Two things follow. First, coffee does make the stomach produce acid. Second, caffeine is not the main reason, because decaf did almost as much. A later review of coffee and the digestive system by Boekema and colleagues[2] confirmed that coffee stimulates gastrin, the hormone that drives acid secretion, and gastric acid output.
What does not follow is that this acid is harmful. The stomach is built to produce and contain strong acid; it does so every time you eat. The question is whether coffee's extra acid, particularly without food to buffer it, causes damage or symptoms. That needs different evidence.
One more detail on acid is worth knowing, because it runs against intuition. Rubach and colleagues[3] gave nine healthy volunteers two coffees with similar caffeine content, a medium-roast market blend and a dark roast blend, and measured stomach acid. The dark roast contained more N-methylpyridinium, a compound created during roasting, and much less chlorogenic acid, 323 mg per litre compared with 1,126 in the medium roast. It stimulated gastric acid secretion less effectively than the medium roast.
That is a tiny study, and it measured acid rather than comfort, so it is a lead rather than a rule. But it is a useful correction to the idea that a darker, more bitter coffee must be rougher on an empty stomach. If anything, this evidence hints the reverse, and the chemistry behind that is covered in our article on how acidic coffee really is.
Claim two: coffee on an empty stomach causes ulcers
Peptic ulcers, sores in the lining of the stomach or the first part of the small intestine, were once thought to be caused mainly by stress and acid-producing foods. That view changed dramatically once the bacterium Helicobacter pylori was recognised as a major cause, alongside anti-inflammatory painkillers such as ibuprofen and aspirin. Coffee's reputation for causing ulcers mostly predates that shift.
A large study by Shimamoto and colleagues in PLoS One[4] tested coffee directly. They examined 8,013 healthy Japanese adults by endoscopy, 5,451 of whom drank coffee and 2,562 of whom did not, and looked for gastric ulcers, duodenal ulcers, reflux oesophagitis and non-erosive reflux disease. They adjusted for age, sex, body weight, H. pylori infection, a marker of stomach acid production, smoking and alcohol.
Coffee was not significantly associated with any of the four conditions. The factors that did matter included H. pylori infection and smoking for ulcers. The team also ran a meta-analysis of earlier studies and found no association between coffee and either gastric or duodenal ulcers. This was a cross-sectional study, so it cannot rule out every possibility, but it is strong evidence against coffee as an important cause of ulcers.
Claim three: coffee causes indigestion and stomach pain
Many people feel queasy or uncomfortable after coffee without food, and that experience is real for them. Whether coffee causes the chronic condition called functional dyspepsia, persistent upper abdominal pain, fullness or early satiety without an obvious cause, is a separate question. Koochakpoor and colleagues, in Scientific Reports[5], studied 3,362 Iranian adults and found that 14.5% had functional dyspepsia. After controlling for confounders, neither coffee, with an odds ratio of 1.27 that was not statistically significant, nor caffeine was associated with dyspepsia or its individual symptoms, including epigastric pain.
That fits with the earlier review by Boekema and colleagues[2], which found no association between coffee and dyspepsia, even though heartburn was the symptom most commonly reported after coffee. These studies did not look specifically at whether coffee was drunk with or without food, which is a real gap. But they suggest that for most people, coffee is not a cause of long-term stomach trouble.
Claim four: morning coffee spikes cortisol
Cortisol is a hormone involved in the stress response and in regulating energy. It naturally peaks shortly after waking. The claim online is that coffee on an empty stomach, especially first thing, drives cortisol dangerously high. Caffeine does raise cortisol, but the details matter.
A carefully controlled trial by Lovallo and colleagues in Psychosomatic Medicine[6] tested 96 healthy men and women over four weeks. After five days without caffeine, a caffeine challenge caused a robust increase in cortisol across the day. But after five days of taking 300 mg or 600 mg of caffeine a day, the cortisol response to the first 9 am dose disappeared entirely. Cortisol did rise again in the afternoon after a second dose, and then returned to normal in the evening. The authors concluded that daily caffeine reduces, but does not eliminate, the cortisol response.
The design of that trial is worth appreciating, because it explains why earlier warnings about caffeine and stress hormones were overstated. Most earlier studies tested caffeine in people who had abstained from it, which is exactly the condition that produced the biggest response here. Real coffee drinkers are rarely in that state. The finding that the afternoon dose still raised cortisol also matters: tolerance was partial, and it seemed strongest for the first dose of the day after a night without caffeine.
In practical terms, the morning cortisol effect is largest in people who do not normally drink caffeine, and it largely vanishes for the first cup in habitual drinkers. A separate trial by Gavrieli and colleagues in the Journal of Nutrition[7] gave 16 healthy men caffeinated coffee, decaf or water with a breakfast snack. Caffeinated coffee kept cortisol from falling as it normally would through the morning, with levels higher than the control from 60 minutes onward. But it had no effect on appetite, energy intake, blood glucose, insulin or inflammation markers. A temporary rise in cortisol within the normal daily range is not the same as chronic stress.
Claim five: coffee before breakfast worsens blood sugar
This claim has the most interesting recent evidence behind it. Smith and colleagues, in the British Journal of Nutrition[8], ran a randomised crossover trial in 29 young, healthy adults. Each person did three oral glucose tolerance tests, a standard sugary drink followed by blood sampling. One test followed a normal night's sleep. One followed a night in which they were woken every hour. And one followed the same broken night, with a strong black coffee containing about 300 mg of caffeine drunk 30 minutes before the sugary drink.
The broken sleep on its own did not change the blood sugar response. But the coffee did. Peak blood glucose was 8.20 mmol/l after normal sleep, 8.23 after broken sleep, and 8.96 after broken sleep plus coffee. Peak insulin was also highest after coffee, at 310 pmol/l. The authors concluded that if a strong caffeinated coffee is drunk before the morning's first sugar load, a reduction in glucose tolerance can be expected.
This is a real, measurable effect, and it fits with earlier research showing that caffeine can temporarily reduce insulin sensitivity. It needs to be kept in proportion, though. The study used a large dose of caffeine, a pure glucose drink rather than an ordinary breakfast, and young healthy volunteers. It measured one morning, not long-term health. And it sits alongside a large body of observational research linking habitual coffee drinking to a lower long-term risk of type 2 diabetes, which we cover in our article on coffee and diabetes. The short-term and long-term effects are different phenomena.
Does eating first change anything?
Surprisingly, the studies above give only indirect answers, because almost none of them compared the same coffee with and without food. What they do show is that food changes the context. In the reflux experiment by Wendl and colleagues[9], regular coffee still caused more acid reflux than water even when it was drunk with a standard breakfast, so eating does not cancel coffee's effects on the upper gut. In the appetite trial by Gavrieli and colleagues[7], coffee drunk with a breakfast snack did not change blood sugar or insulin over the following three hours, which contrasts with the glucose-drink study below, although the doses, foods and people were different.
There is also one situation where drinking coffee away from food is arguably better. Coffee reduces the absorption of iron from a meal eaten with it, and in the experiment by Morck, Lynch and Cook[10], that inhibition did not occur when the coffee came an hour before the meal, while coffee an hour after the meal inhibited absorption as much as coffee drunk with it. For anyone who struggles with low iron, that is one of the few concrete, evidence-based reasons to prefer coffee before breakfast rather than with or straight after it. That trade-off is explained in our article on coffee after meals and iron absorption. So the popular advice to never drink coffee before eating can conflict with the equally popular advice not to drink it with meals.
Putting the claims together
Of the five common claims, one is clearly true but not obviously harmful: coffee stimulates stomach acid. One is clearly not supported: coffee is not an important cause of ulcers. One has little support at the population level: coffee does not appear to cause chronic indigestion, although individuals can react to it. One is true but overstated: caffeine raises cortisol, but mostly in people who are not used to it. And one is true in the short term with an important caveat: a strong coffee before a sugary breakfast can raise the blood sugar response that morning.
It is also worth noticing how often the scary version of each claim depends on studying people in an unusual state: abstaining from caffeine before a cortisol test, drinking a pure glucose solution after being woken every hour, or taking caffeine on its own rather than as coffee. Those designs are good science for isolating a mechanism. They are a poor guide to what an ordinary morning cup does to a habitual drinker who slept normally.
The limits of the evidence
Surprisingly little research has compared coffee with food against coffee without food directly, which is the question most people are asking. The studies above mostly tested coffee in general, or caffeine before a standard test drink. The ulcer and dyspepsia studies are cross-sectional, showing associations at one point in time. The cortisol and blood sugar trials are small, short, and done in healthy young adults. People with existing stomach conditions, diabetes, or those taking medication were not the focus of these studies.
A practical reading
For most healthy people, there is no strong evidence that coffee before breakfast is harmful. If it makes your stomach uncomfortable, that is a good enough reason to have something to eat first, whatever the population studies say. If you have diabetes or prediabetes, the blood sugar finding is worth discussing with your doctor, particularly if your morning coffee comes before a high-carbohydrate breakfast. And if you are relying on coffee after a terrible night's sleep, the one clear lesson from the glucose study is that it may make that morning's breakfast hit your blood sugar a little harder.
Coffee's effects on heartburn are covered in our article on coffee and acid reflux, and its effect on the bowel in the article on why coffee makes you need the toilet. Nothing here is medical advice.